Thermionic cathode for electron beam apparatus

ABSTRACT

A thermionic cathode comprising a lanthanum hexaboride (LaB6) emitter and an emission current-intercepting plate made of rhenium containing an aperture disposed adjacent the emitter. The separation between the emitter and plate is set as small as possible without physical contact between them, typically 0.1 mm. The beam emitted through the aperture has high brightness and low energy spread. To prevent corrosion of the emitter it is housed in a cavity which is exposed to the vacuum of the apparatus only through the aperture.

United States Patent [72] Inventor Karl H. Loefller 3,462,635 8/1969 Broers 313/317 Sen .10 fi- FOREIGN PATENTS 25$- 2:5 1970 1,148,708 12/1957 France 313/330 [45] Patcme M'zs 1971 852,734 ll/l960 Great Britain 313/330 73 Assign I -fl w Buflnm M dfl Primary Examiner-David Schonberg C r fl Assistant Examiner-Paul A. Sacher Armonk, N.Y. Att0rneysl-lanifin and Jancin and Melvyn D. Silver [54] THERMIONIC CATHODE FOR ELECTRON BEAM APPARATUS 18 Claims, 2 Drawing Figs.

[52] U.S.Cl 33113423111, ABSTRACT; A thermionic cathode comprising a hmhanum hexaboride (LaB emitter and an emission current-interce tm 1/15 ing plate made of rhenium containing an aperture disposed :d-

"GUN/08,1401]. 1/14 jacent the emitter. The separation between the emitter and 0 Search u plate is set as small as possible wihout physical ontact 337 between them, typically 0.1 mm. The beam emitted through [561 Mums CW :1:viii'il'io fin iZZ'Lilfiffiliiilfi'ifi'fiifiifillfi UNITED STATES PATENTS is exposed to the vacuum of the apparatus only through the 3,054,962 9/1962 Opitz 313/337 X aperture.

2 QQQ 12 12 r mqxs: \wccqsqg :1 Q \\\\\10 110\ ll L 4 PATENTEU UEB28 I97! FIGJ KARL H. LOEFFLER W BMSM AGENT THERMIONIC CATHODE FOR ELECTRON BEAM APPARATUS BACKGROUND OF THE INVENTION This invention relates to apparatus for producing electron beams. In particular, this invention concerns a thermionic cathode for electron beam apparatus which produces an electron beam having low energy spread and high brightness.

Present thermionic electron beam guns are designed to provide high brightness only, and have then typically a high energy spread. The combination of low energy spread and high brightness has not yet been achieved by others working in this art.

The term energy spread refers, as is well known, to the variation in energy among the electrons discharged from the emitter of the cathode. High energy spread is undesirable as it causes chromatic aberration of the beam when the beam is focused by the magnetic lens elements of the gun. The more energetic electrons are less affected by the magnetic lens elements than the less energetic electrons. This effect reduces the resolving power of the electron beam apparatus.

In many applications, deflection of the beam as well as focusing is required; and high energy spread results in a second aberration mode. This aberration, commonly termed chromatic aberration of the deflection system, occurs because the deflection yoke also has less effect on the more energetic electrons than on the less energetic ones.

The prior art has achieved solutions to these problems which are less than completely satisfactory. Up to the present, in fact, the principal solution has been to reduce the brightness of the beam. Brightness is defined as the electron emission current density per solid angle. By reducing the brightness, the energy spread may be reduced sufficiently to yield a beam with adequate resolution. For instance, in certain devices such as the electron microscope, high resolution is critical but brightness is generally less so. Hence in the electron microscope, brightness is sacrificed to optimizing the energy spread. This is accomplished by reducing the number of electrons emitted. In thermionic electron gun devices requiring high brightness it has been impossible to achieve a narrow energy spread, i.e., high resolution, and the art has generally been resigned to designing around this problem rather than to solving it.

From the foregoing, it is evident that the practice in this field has been to trade off resolution and high electron emission (brightness), depending on the application. However, in applications such as electron beam recording apparatus and in fabricating conductive bands on semiconductor circuits, the combination of high resolving power and brightness is of major importance. In these areas electron beam apparatus often compete with devices from other arts. In semiconductor manufacture, for example, optical exposure and removal techniques are predominant because they are capable of providing high throughput and adequate resolving power. Electron beam apparatus, on the other hand, theoretically offer high resolving power; but throughput, which is proportional to the brightness of the beam, is low.

In this invention, the use of lanthanum hexaboride as an emitter material in a novel cathode configuration provides an electron beam having high brightness and low energy spread characteristics. It has long been known that rare earth borides, in particular lanthanum hexaboride, have properties which make it highly suitable as a cathode material. It is an exceptionally good emitter, has an attractively high ratio of emission-current density to evaporation rate, a good tolerance for cold exposure of atmospheric pressure and its resistance to poisoning compares favorably with most other emitter materials.

The characteristic which has prevented the wide-spread use of lanthanum hexaboride as an emitter material is its highly reactive nature at the high temperatures at which it operates. Most active gases will react with LaB at elevated temperatures, particularly oxygen, causing corrosion which can make it useless before its evaporation-limited normal end of life. For example, at typical emission current densities of a heated LaB emitter, its lifetime is limited by corrosion rather than by evaporation if the residual air pressure at the emitter exceeds about l0 torr. Almost all of the oxygen molecules arriving at the emitter surface react with the LaB consistent with the well known high formation energies of boric oxides. In comparison, tungsten emitters react with only 1 percent or less of the oxygen molecules arriving at its surface.

The problem of high reactivity of LaB is aggravated in electron beam apparatus which operate under conditions of relatively poor vacuum, such as demountable systems, or in systems where materials are passed into and out of the vacuum chambers which cause gases to be produced.

Assuming solution of the LaB, corrosion problem, however, its use in conventional cathode configurations still will not provide an electron beam with high brightness and low energy spread. In fact, at the present time, there is no thermionic cathode structure known to the instant inventor which will provide an electron beam with high brightness and low energy spread.

SUMMARY OF THE INVENTION It is therefore an object of this invention to combine low energy spread with high brightness in an electron beam.

It is a further object of this invention to enable the use of electron beam apparatus economically in operations requiring high resolving power and throughput.

It is another object of this invention to reduce substantially the corrosion rate of lanthanum hexaboride when it is used as an emitter in electron beam apparatus.

These and other objects are achieved by providing a thermionic cathode in an electron beam apparatus comprising an emitter fabricated from lanthanum hexaboride (LaB,) and an emission current-intercepting plate made of rhenium disposed adjacent to the emitter. The emitter and the plate are as close as physically possible to each other without physical contact, that is, maintaining some spaced relationship. The actual spacing is to within state-of-the-art limits. The plate contains an aperture to pass the electron beam. In the preferred embodiment, the aperture is made as small as possible, again, the actual spacing being within state-of-the-art limits.

This structure produces an electron beam having high brightness and low energy spread, apparently because it ensures that the effective crossover diameter of the beam is very small. Most of the unused current from the emitter is intercepted by the positively biased intercepting plate located as close as possible to the emitter surface, thereby preventing interactions between individual beam electrons which increase the energy spread of the beam. The practical minimum emitter-plate gap size, around 0.1 mm., permits the rhenium plate potential to be kept very low, thereby reducing emitter damage from ion bombardment.

In the preferred embodiment, the corrosion rate of the Lab,-, emitter is substantially reduced by partially isolating the emitter from the gun vacuum. The emitter is enclosed in a housing and the only connection from the emitter cavity to the gun vacuum is through the plate aperture.

Another feature of the invention lies in the shape of the rhenium emission-current-intercepting plate. To ensure that there is no possibility of contact between the emitter and the plate through thermal expansion of the plate, it is bulged slightly away from the emitter. The plate then expands in the direction of the bulge when heated.

Still another feature of this invention is the shape of the emitter when it is designed to be directly heated rather than indirectly heated. The directly heated emitter is substantially in the shape of an elongated U. The parallel, vertical legs of the emitter are in very close proximity, preferably around 15 mils, thereby preventing undue thermal stresses on the emitter.

The principal inventive structure and its effects willallow the grid-to-anode gap in an electron beam apparatus to be substantially greater than in apparatus using conventional cathodes. This prevents or drastically reduces high-voltage arcing between the cathode and anode.

BRIEF DESCRIPTION OF THE DRAWING The invention will be more fully understood by referring to the following detailed description taken in connection with the accompanying drawings, forming a part thereof, in which:

FIG. 1 is a cross-sectional view of the emitter-unit assembly containing the thermionic cathode of the present invention.

FIG. 2 is a greatly enlarged cross-sectional view of the inventive thermionic cathode.

Referring now to FIG. I there is shown the overall structure of the-emitter unit assembly of this invention. A lanthanum hexaboride emitter I is illustrated which is located within a cavity 3. The cavity is formed by a heat conducting but electrically insulating ceramic base 2, annular insulating ceramic collar 4, emission current-intercepting plate 6 and conductive pins l{0. Rhenium plate 6 contains an aperture for transmitting the electron beam current, the aperture being the sole connection between emitter 1 and the vacuum ambient of the electron beam apparatus. As can be seen from FIG. I, the LaB emitter l is generally in the shape of an elongated U with laterally projecting tabs 11 and a neck portion 7 at the base projecting toward plate 6. The preferred shape of the emitter and the rhenium plate will be more fully described below with reference to FIG. 2. The laterally projecting tabs 11 of emitter l are brazed to pins which provide a connection to a power source for heating the emitter, thereby causing it to emit electrons. The conductive pins are a refractory metal, preferably molybdenum.

Surrounding ceramic base 2 is a heating foil 12, functioning as a means to provide heat to the walls of cavity 3 for outgassing the cavity prior to operating the emitter if it has been exposed previously to atmospheric pressure. If there are contaminants present in the cavity, corrosion of the LaB, emitter would occur because the emitter itself would produce outgassing and would simultaneously be corroded by the released (outgassed) gases. Heating foil 12 is preferably made of rhenium and is connected to a power source, not shown, for supplying approximately 30 watts power input.

Grid means 14 and anode means 16 for accelerating the beam are connected to sources of positive potential, not shown. The distance between the grid and anode is around 40 mm. in the preferred embodiment. This is much greater than the distance in standard electron beam guns and its significance will be discussed in greater detail below.

In practice, grid 14 and anode 16 may be, for example, biased around +500 volts and kilovolts, respectively, with respect to the emitter 1. At these values, the grid will provide virtually no focusing effect on the beam; the beam configuration is determined by the lateral thermal spread of the electron trajectories in the approximately homogeneous accelerating field between the aperture of plate 6 and anode 16.

As already noted, emission current-intercepting plate 6 is made of rhenium. This metal is required because it is chemically insensitive to the evaporation and corrosion products of LaB and is highly refractory so that it withstands the high operating temperature (approximately 1,000 C. or greater) without significant changes in its microgeometry. Rhenium also has a low thermal conductivity, reducing overall heating power requirements for the cathode. Other refractory metals which a skilled person would think could be used in place of rhenium, such as tungsten or molybdenum, are not suitable.

At the high operating temperatures involved both tungsten be understood that the LaB emitter of this inventive thermionic cathode could be indirectly heated. One such indirectheating apparatus which could be used with good results is described in Us. Pat. No. 3,462,635 of A. N. Broers and assigned to the same assignee as the present invention.

Referring now to FIG. 2, a greatly enlarged view of the cathode illustrates the invention more clearly. The directly heated emitter is a flat piece of LaB,, preferably around l5 mils thick and substantially in the shape of an elongated U with laterally projecting tabs 11, legs 9 which join at base 15, and a neck portion 7 at the base 15 projecting toward plate 6. Power supply 20 supplies heating current through conductive pins 10 for the emitter.

The important sections of the emitter from the standpoint of thermal stress are the contact joints between tabs 1 l and conductive pins 10. If the emitter had the standard hairpin shape, it would tend to crack at the normal operating temperature of l500-1600 C. because of its hard, brittle, ceramiclike nature. It has been found that this problem can be completely avoided if the legs 9 of the emitter are nearly perfectly parallel and if the .gap 17 between them is made very small. In the preferred embodiment, the width of gap 17 equals the width of one of the legs, which is 15 mils. With this construction, there is negligible thermal expansion between legs 9,9. Rather, as the emitter is heated to its operating temperature, the expansion occurs downward toward plate 6.

The particular shape of the base 15 and projecting neck 7 of emitter l is not very important. It is preferable that the tip of portion 7 facing plate 6 be substantially flat. However, for certain applications projecting portion neck 7 could be omitted completely.

Emission current-intercepting plate 6 is shown having an aperture 8 and connected to a source of positive potential 18. Rhenium plate 6 is located as close as possible to the tip of emitter 1 without there being physical contact between them. At the present state of the art, the gap is around 0.1 mm. Projecting portion 7 is disposed adjacent to and in and alignment with aperture 8 of plate 6. In the preferred embodiment the size of the aperture is as small aspossible within the limits of the art. For practical purposes, the aperture 8 comprises a conical hole having a minimum diameter of from 5 to 10 microns in plate 6 which has a thickness of 25 microns. It will be seen in FIG. 2 that the preferred plate embodies a bulged portion adjacent projecting portion 7, the bulge directed outward from the tip of portion 7. Aperture 8 is substantially in the center of the bulge.

The preferred embodiment of the cathode illustrated in FIG. 2 is principally responsible for the vastly improved characteristics of the electron beam.

Of prime importance is the minimum plate-emitter gap. This ensures that any unused emission current from the emitter is intercepted as close as possible to the emitter surface, substantially reducing the energy spread as compared to conventional tungsten and rare-earth boride cathodes. The minimum gap size also means that the potential 18 supplied to plate 6, which generates a field for overcoming space charge effects at high current densities, may be low -from I00 to volts. This potential is thereby at or below the effective sputtering threshold for ions, reducing or eliminating emitter damage from ion bombardment inside the cavity. The effective sputtering becomes significant. It also prevents any arcing between emitter and plate.

The minimum diameter of the aperture 8 also contributes very significantly to the reduced energy spread of the beam. For reasons of clarity, the relationship of the width of projecting neck 7 and aperture 8 is not obvious from viewing FIG. 2. In practice however, neck 7 will be 15 mils in width, which is an order of magnitude larger than the 5-10 micron diameter of aperture 8 through which the beam current passes. As has already been discussed, neck 7 might be eliminated altogether. In that case the entire surface of emitter base 15 would emit electrons to be intercepted by plate 6 or passed through aperture 8.

Besides contributing to the reduction of energy spread, the minimum size of the aperture 8 produces a special benefit for the LaB emitter. It will be recalled that in FIG. I, the emitter is completely enclosed in cavity 3 and exposed to the gun vacuum only through beam-transmitting aperture 8. Any active gases, which initially may be present in cavity 3, which are primarily oxygen and oxygen-containing compounds as e.g. water, are rapidly consumed in a corrosion process with the LaB,,. This corrosion effect has been a major deleterious factor in the use of LaB, as emission material. However, in the present invention. once the active components of the residual air and other gases within the cavity is consumed by the emitter, virtually no further corrosion takes place as compared to prior-art configurations. During the high-temperature operation of the emitter the corroding gases will be consumed by the emitter, forming corrosion byproducts which evaporate from the emitter to the cavity walls. The pumping" effect reduces the partial pressures of the active gases to an insignificant level.

In the operation of the apparatus, referring now to both FIG. 1 and FIG. 2, heat supplied by foil 12 through ceramic base 2 is first used to outgas cavity 3 after it has been exposed to atmospheric pressure as from a previous operation. After this step, current of approximately 7-9 amperes supplied through pins provides approximately 7-l 0 watts to heat the LaB emitter to its operating temperature of 1500-l 600 C. at a typical vacuum range of l0 to 10 torr. The intercepting rhenium plate 6 is biased at +100 volts or more with respect to the emitter and draws emission current of up to 40 ms. Useful beam current of several microamperes passes through aperture 8.

During the operation, the rhenium intercepting plate 6 will increase in temperature because of the high temperature of the emitter. If the plate were designed to be perfectly flat, its thermal expansion might cause it to bulge toward the tip emitter neck 7, reducing the plate-emitter gap. The gap reduction would then increase the emission current, which is partially space-charge limited. This current increase would then cause a further expansion of the plate until it touched the emitter, resulting in a short circuit. The short circuit would cool the plate, contracting it away from the emitter, initiating a short circuit open circuit cycle. This effect can be corrected, of course, by locating the intercepting plate sufficiently far from the emitter to compensate for the thermal expansion. This solution is effective but not very desirable because a main thrust of the inventive concept is to make the plate-emitter gap as small as possible. A better solution, one which is preferred in this embodiment, is to shape the plate to bulge slightly around the emitter tip, the bulge directed away from the emitter tip. It has been found that this design not only reduces the axial thermal expansion of intercepting plate 6 but also directs any expansion due to heat away from the emitter tip.

The plate surface around the aperture is heated to over l,000 C. by the intercepted emission current. This is a required design feature causing the evaporation and corrosion products to migrate very rapidly toward cooler portions of the plate, thus keeping the aperture clean. The operating temperature of the plate, on the other hand, should be low enough to prevent an excessive recrystallization rate of the plate material, thus maintaining its original shape and size over long periods of operation.

Returning to the operation of the apparatus, the beam passing through the aperture 8 is accelerated through grid 14 to anode 16. The biasing potentials of grid 14 and anode l6, supplied through biasing means not shown, are for example, +500 volts and +15 kilovolts, respectively. At these levels, the grid has essentially no focusing effect and the beam configuration is determined by the lateral thermal spread of the electron trajectories in the approximately homogeneous accelerating field between aperture 8 and anode 16. Under these operating conditions, only the anode opening 22 exerts its weak, negative focal power, reducing the virtual source diameter to a value about 30 percent less than the true diameter ofthe aperture 8.

The gap between the grid 14 and anode 16 may be made an order of magnitude larger than the gap in ordinary electron beam apparatus. Ordinary electron guns have to minimize the gap, usually to around 4 mm., in order to achieve a sufficiently high field strength at the emitter to overcome space-charge effects which limit emission current. In the present invention, on the other hand, the field strength is generated by the rhenium plate at a low voltage where vacuum arcs cannot be generated or sustained. This beam formation mechanism allows one to choose the grid-anode gap to be as large as desirable to eliminate high voltage arcing from anode to grid. In the preferred embodiment the gap is approximately 40 mm.

At a grid voltage of+l 70 v., the gun operates in a telefocus mode and between 20 to 50 v., the beam current drops from its full value to zero due to reflection of the electrons at the potential minimum created by the grid. The angular distribution of the emitted beam exhibits a structure caused by patches of low work function on the emitter surface. On the other hand, the current-density distribution in the aperture plane appears to be constant within :1 percent. At grid voltages above v., secondary electrons generated at the aperture can pass the grid and add about 20 percent to the net beam current, but only a negligible I to 2 percent to the axial brightness.

A typical set of operating conditions and measured performance data are as follows: anode potential, +15 kv.; grid potential, +500 volts; rhenium plate potential, +l0l volts; plate aperture diameter, 7.2 microns; gap between emitter and plate, 0.1 mm.; cavity pressure, 2.4 X 10 torr; estimated emitter temperature, l,575 C. for approximately saturated emission; energy spread (AV 0.58 volts; brightness 2 X 10 a./cm. sr; net beam current 2.7 microamperes plate current= 33 milliamperes.

The observed energy spread of AV 0.58 v. is measured with a retarding-potential analyzer and is in reasonable agreement with a theoretically estimated energy spread of (AV 0.63 v. for the above operating conditions. Energy spread measurements of drastically lower accelerating potentials resulted in only an insignificant reduction of the energy spread, thus indicating that at higher anode voltages, for example 75 kv., an energy spread of AV 0.6 v. can be expected at a brightness which would then equal 10 a./cm. sr.

Accelerated life testing on the thermionic cathode of this invention as compared to conventional tungsten hairpin cathodes demonstrate the superior characteristics of this inventive structure. At a power input to the rhenium plate of 4.0 watts and pressure increased to 5 X 10' torr of air, the plate shows pinhole formation caused by oxidation of the rhenium after 200 hours of operation. The LaB emitter is still usable. Conventional tungsten hairpin cathodes fail after less than 20 hours under these same conditions.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A thermionic cathode in an electron beam apparatus for supplying an electron beam with low energy spread and high brightness comprising:

a lanthanum hexaboride emitter;

a rhenium plate disposed as close as possible to a face of said emitter, but not in physical contact therewith, means for biasing said plate positively with respect to said emitter, to allow attracting the emission current, said plate having an aperture to enable the flow therethrough of electrons to form a beam during operation of said cathode.

2. A thermionic cathode as in claim 1 wherein the aperture is between substantially 5 and 10 microns in diameter.

3. A thermionic cathode as in claim 1 wherein the distance between said emitter and said plate is approximately 0.1 mm.

4. A thermionic cathode as in claim 1 wherein said rhenium plate embodies a section bulged away from said emitter, the aperture being substantially in the center of said bulged section, thereby maintaining a gap between said emitter and said plate when said plate undergoes thermal expansion.

5. A thermionic cathode as in claim 1 wherein the bias on said rhenium plate is below the effective sputtering threshold of ions, thereby reducing emitter damage from ion bombardment.

6. A thermionic cathode as in claim 5 wherein said plate potential is approximately 100 volts.

7. A thermionic cathode as in claim 1 including a housing enclosing said emitter within a cavity, said rhenium plate forming a section of said housing, the aperture of said plate being the sole connection between the vacuum of the electron beam apparatus and the cavity. 1

8. A thermionic cathode as in claim 7 wherein the aperture has a diameter of microns or less.

9. A thermionic cathode as in claim 8 wherein the aperture is between substantially 5 and 10 microns in diameter.

10. A thermionic cathode as in claim 7 including heating means for outgassing the cavity after it has been exposed to contaminant material.

11. A thermionic cathode as in claim 1 further comprising current means connected to said emitter for directly heating said emitter.

12. A thermionic cathode as in claim 11 further comprising a housing enclosing said directly heated emitter in a cavity, said rhenium plate forming a section of said housing, and wherein the aperture of said plate has a diameter of 10 microns or less.

13. A thermionic cathode as in claim 12 wherein said emitter is substantially in the shape of an elongated U, having vertical legs and a base, said base disposed adjacent said plate aperture, the upper portions of said vertical legs connected to said current means, said legs nearly perfectly parallel and in close proximity with each other, thereby preventing undue thermal stress on said emitter when it is brought to its operating temperature.

14. Electron beam apparatus for supplying an electron beam with low energy spread and high brightness comprising:

a lanthanum hexaboride emitter;

a rhenium plate disposed as close as possible to a face of said emitter, but not in physical contact therewith,

means for biasing said plate positively with respect to said emitter to allow attracting the emission current, said plate having an aperture to enable the flow therethrough of electrons to form a beam during operation of said apparatus, I

grid means spaced from said plate beyond the aperture,

means for biasing said grid means positively with respect to said emitter; and

anode means spaced from said grid means on the side opposite said plate for attracting the beam, the spacing between said grid means and said anode means being sufficiently large to prevent arcing therebetween.

15. Electron beam apparatus as in claim 14 wherein the aperture has a diameter of 10 microns or less.

16. Electron beam apparatus as in claim 15 wherein the biasing potentials with respect to said emitter, said anode means, said grid means and said rhenium plate are, respectively: greater than 15 kv., greater than 170 volts, and between volts and volts.

17. Electron beam apparatus as in claim 14 wherein the spacing between said grid means and said anode means is approximately 40 mm.

18. Electron beam apparatus as in claim 16 wherein the potentials on said anode means, said grid means and said rhenium plate are, respectively: approximately l5 kv., approximately 500 volts and approximately 100 volts. 

2. A thermionic cathode as in claim 1 wherein the aperture is between substantially 5 and 10 microns in diameter.
 3. A thermionic cathode as in claim 1 wherein the distance between said emitter and said plate is approximately 0.1 mm.
 4. A thermionic cathode as in claim 1 wherein said rhenium plate embodies a section bulged away from said emitter, the aperture being substantially in the center of said bulged section, thereby maintaining a gap between said emitter and said plate when said plate undergoes thermal expansion.
 5. A thermionic cathode as in claim 1 wherein the bias on said rhenium plate is below the effective sputtering threshold of ions, thereby reducing emitter damage from ion bombardment.
 6. A thermionic cathode as in claim 5 wherein said plate potential is approximately 100 volts.
 7. A thermionic cathode as in claim 1 including a housing enclosing said emitter within a cavity, said rhenium plate forming a section of said housing, the aperture of said plate being the sole connection between the vacuum of the electron beam apparatus and the cavity.
 8. A thermionic cathode as in claim 7 wherein the aperture has a diameter of 10 microns or less.
 9. A thermionic cathode as in claim 8 wherein the aperture is between substantially 5 and 10 microns in diameter.
 10. A thermionic cathode as in claim 7 including heating means for outgassing the cavity after it has been exposed to contaminant material.
 11. A thermionic cathode as in claim 1 further comprising current means connected to said emitter for directly heating said emitter.
 12. A thermionic cathode as in claim 11 further comprising a housing enclosing said directly heated emitter in a cavity, said rhenium plate forming a section of said housing, and wherein the aperture of said plate has a diameter of 10 microns or less.
 13. A thermionic cathode as in claim 12 wherein said emitter is substantially in the shape of an elongated U, having vertical legs and a base, said base disposed adjacent said plate aperture, the upper portions of said vertical legs connected to said current means, said legs nearly perfectly parallel and in close proximity with each other, thereby preventing undue thermal stress on said emitter when it is brought to its operating temperature.
 14. Electron beam apparatus for supplying an electron beam with low energy spread and high brightness comprising: a lanthanum hexaboride emitter; a rhenium plate disposed as close as possible to a face of said emitter, but not in physical contact therewith, means for biasing said plate positively with respect to said emitter to allow attracting the emission current, said plate having an aperture to enable the flow therethrough of electrons to form a beam during operation of said apparatus, grid means spaced from said plate beyond the aperture, means for biasing said grid means positively with respect to said emitter; and anode means spaced from said grid means on the side opposite said plate for attracting the beam, the spacing between said grid means and said anode means being sufficiently large to prevent arcing therebetween.
 15. Electron beam apparatus as in claim 14 wherein the aperture has a diameter of 10 microns or less.
 16. Electron beam apparatus as in claim 15 wherein the biasing potentials with respect to said emitter, said anode means, said grid means and said rhenium plate are, respectively: greater than 15 kv., greater than 170 volts, and between 100 volts and 150 volts.
 17. Electron beam apparatus as in claim 14 wherein the spacing between said grid means and said anode means is approximately 40 mm.
 18. Electron beam apparatus as in claim 16 wherein the potentials on said anode means, said grid means and said rhenium plate are, respectively: approximately 15 kv., approximately 500 volts and approximately 100 volts. 